The present invention relates to a method for assessing the degradation of oxide-forming coatings on components after a period of operation of the component, in particular to a method for assessing degradation of oxide-forming coatings on gas-turbine blades in, for example, aerospace, land-based and marine applications.
Oxidation of the surface of components is a problem that may critically affect operational performance in many different applications. The problem of oxidation is particularly severe in the case of gas-turbine blades, notably in high-temperature aerospace applications, where the base alloys commonly used for the blades may exhibit relatively poor oxidation resistance.
Coating the surface of the relevant component with a protective oxide-forming coating can reduce the problem of oxidation.
Generally speaking, such oxide-forming coatings provide a “reservoir” of an oxide-forming element, such as Aluminum, which ideally forms a stable, continuous, adherent and slow-growing oxide layer on the exposed surface of the coating during operation of the component, in order to protect the underlying component substrate. The oxide-layer is maintained during operation of the component by diffusion of the oxide-forming element into the oxide layer, but this also has the effect of depleting the reservoir of available oxide-forming element in the remainder of the coating. The reservoir provided by the remnant coating is typically also depleted by diffusion of the oxide-forming element into the component substrate, which diffusion forms a secondary inter-diffusion layer between the remnant coating and the unaffected part of the component substrate. Thus, the coating degrades over the course of its life until the reservoir of oxide-forming element in the remnant coating layer becomes exhausted and the coating is unable to maintain the oxide layer, at which point coating failure is likely to occur.
It is commonly the case that the life of an oxide-forming coating is shorter than the operational life of the component itself, and therefore it is preferable to re-coat most components at least once during their life in order to continue to prevent significant oxidation of the surface of the components. On the other hand, in the interest of cost-saving, it is undesirable to prematurely re-coat components. A decision as to whether or not to re-coat a component must balance these competing factors and is therefore normally based on an assessment of the degradation or “life-usage” of the coating.
Conventionally, an assessment of the degradation of the coating is typically carried out via visual methods or so-called “cut-up” methods; however, both of these methods have drawbacks.
In the case of visual methods, these rely on a visual assessment of the exterior of the component and, consequently, the assessment is qualitative by nature and subject at least in part to human judgement and error. Due to the potentially severe consequences of under-estimating the amount of coating degradation, for example in aerospace applications, qualitative human estimates tend to be conservative in nature, which results in premature re-coating and/or scrapping of components.
In the case of “cut-up” methods, the component is cut into sections and the cross-sectional microstructure of the coating is examined. However, due to the destructive nature of the assessment, the assessment must be carried out on a representative sample taken from a component or components, which means that the assessment is prone to statistical inaccuracies. In addition, preparation and examination of the sections is both time-consuming and costly.